1. Field of the Invention
The invention relates to signal generators and methods for generating signals, which may be employed by image sensors. More particularly, one or more aspects of the invention relate to complimentary metal oxide semiconductor (CMOS) image sensors and methods of operating CMOS image sensors having improved noise reduction and/or elimination properties and improved image quality.
2. Description of the Related Art
Image sensors may be employed in various fields, e.g., robotics, transportation, automobiles, satellite-based instrumentation, navigation, etc. Image sensors may include a two-dimensional array of pixels formed on a semiconductor substrate, and such a pixel array may correspond to an image field of an image frame.
Image sensors may include a photoelectric conversion element that is capable of accumulating a quantity of electrical charge corresponding to an amount of detected energy, e.g., visible light, etc. For example, each pixel of a pixel array may include such a photoelectric conversion element and when photons impact a surface of the photoelectric conversion element, free charge carriers may be produced. These free charge carriers may then be collected by the respective photoelectric conversion element. The collected charge carriers may then be converted to an output signal, e.g., a voltage or a current, corresponding to the respective quantity of collected free charge carriers. Each of the pixels of the pixel array may output a respective output signal and each of the output signals may be supplied to an output circuit and employed to generate an image corresponding to the amount of detected energy.
Various types of image sensors are known, e.g., charge coupled device (CCD) image sensors and CMOS image sensors. In comparison to CCD image sensors, CMOS image sensors may be advantageous because, e.g., CMOS image sensors may be fabricated using, e.g., standard CMOS processes, may be integrated with other CMOS devices and circuitry on a single chip enabling miniaturization of devices, may employ relatively low operating voltages, and may consume relatively less power. In comparison to CCD image sensors, however, CMOS image sensors generally need to employ a high resolution analog-to-digital converter (ADC) for converting an analog signal received from an active pixel sensor (APS) to a digital signal.
The quality of image(s) produced by an image sensor may be directly related to a signal to noise (S/N) ratio of the image sensor, e.g., the higher the S/N ratio of an image sensor, the higher the quality, e.g., resolution, of images produced by that image sensor. In an attempt to reduce noise and increase the S/N ratio, CMOS image sensors may employ, e.g., ADCs that perform correlated double sampling (CDS). Such an ADC that also performs CDS may reduce noise characteristics corresponding to aspects of the ADC itself. However, such ADCs that perform CDS may not reduce and/or eliminate other types of noise, e.g., power supply noise generated by a portion(s) of the respective pixel other than the respective ADC.
Demand for image sensors capable of producing higher resolution images is increasing. One reason such demand is increasing is that as chip sizes are decreasing, the negative impact of noise on image quality increases. Therefore, designs and/or structures for image sensors having improved noise reduction and improved image quality are desired.